Man-made structures such as boat hulls, buoys, drilling platforms, dry dock equipment, oil production rigs, and pipes which are immersed in water are prone to fouling by aquatic organisms such as green and brown algae, barnacles, mussels, and the like. Such structures are commonly of metal, but may also comprise other structural materials such as concrete. This fouling is a nuisance on boat hulls, because it increases frictional resistance during movement through the water, the consequence being reduced speeds and increased fuel costs. It is a nuisance on static structures such as the legs of drilling platforms and oil production rigs, firstly because the resistance of thick layers of fouling to waves and currents can cause unpredictable and potentially dangerous stresses in the structure, and, secondly, because fouling makes it difficult to inspect the structure for defects such as stress cracking and corrosion. It is a nuisance in pipes such as cooling water intakes and outlets, because the effective cross-sectional area is reduced by fouling, with the consequence that flow rates are reduced.
The commercially most successful methods of inhibiting fouling have involved the use of anti-fouling coatings containing substances toxic to aquatic life, for example tributyltin chloride or cuprous oxide. Such coatings, however, are being regarded with increasing disfavour because of the damaging effects such toxins may have if released into the aquatic environment. There is accordingly a need for non-fouling coatings which do not release markedly toxic materials.
It has been known for many years, for example as disclosed in GB 1,307,001 and U.S. Pat. No. 3,702,778, that silicone rubber coatings resist fouling by aquatic organisms. It is believed that such coatings present a surface to which the organisms cannot easily adhere, and they can accordingly be called fouling release rather than anti-fouling coatings. Silicone rubbers and silicone compounds generally have very low toxicity. The disadvantage of this anti-fouling system when applied to boat hulls is that although the accumulation of marine organisms is reduced, relatively high vessel speeds are needed to remove all fouling species. Thus, in some instances, it has been shown that for effective release from a hull that has been treated with such a polymer, it is necessary to sail with a speed of at least 14 knots. For this reason silicone rubbers have gained limited commercial success and there is a need for improvement of the anti-fouling and fouling release properties of these environmentally benign coatings.
FR 2 537 985 discloses an anti-fouling coating composition comprising a methyl organosiloxane resin, a silicone elastomer, polytetrafluoroethylene, an acrylic binder, and a solvent or diluent. Since polytetrafluoroethylene is solid at room temperature, this document does not describe a coating composition comprising a fluid fluorinated alkyl-containing polymer or oligomer.
EP 0 903 389 discloses an anti-fouling composition comprising a photocatalytic oxide, a silicone resin or silica, and a water-repellent fluororesin. Tetrafluoroethylene is mentioned as a preferred hydrophobic fluororesin, and in the examples polytetrafluoroethylene particles have been used. This document does not describe a coating composition comprising a fluid fluorinated alkyl-containing polymer or oligomer.